1. Field of the Invention
This invention relates to a control circuit for a hydraulic actuator wherein a pair of hydraulic change-over valves which individually are part of two hydraulic change-over valve sets are operated to cause oil under pressure obtained from them to join each other to operate the hydraulic actuator.
2. Description of the Prior Art
Various methods have been conventionally employed for supplying the entire amounts of discharge pressure oil of two hydraulic pumps, in some cases, in a converging relationship to a single hydraulic actuator to raise the operating speed of the hydraulic actuator. Such methods include a method wherein discharge oil under pressure flows from two hydraulic pumps and are introduced to two independent sets of hydraulic change-over valves to change over two hydraulic change-over valves belonging to the individual hydraulic change-over valve sets at a time so as to obtain a converging oil under pressure flow by which a single hydraulic actuator is operated, or another method wherein discharge oil under pressure from a hydraulic pump which is supplied to one of two hydraulic change-over valve sets is supplied simulataneously to a hydraulic change-over valve belonging to the other hydraulic change-over valve set so as to obtain a large capacity to operate a hydraulic actuator with oil under pressure from the hydraulic change-over valve.
FIG. 2 is a hydraulic circuit diagram illustrating the former method where oil under pressure flows from two hydraulic cylinders for an arm of a hydraulic shovel join each other.
Referring to FIG. 2, the hydraulic circuit shown includes first and second hydraulic pumps 2 and 3 and a pilot pump 4 which are all driven by an engine 1 to draw working oil out of a tank 19 and each provide appropriate discharge oil under pressure by action of a relief valve. Thus, the first hydraulic pump 2 supplies discharge oil under pressure to a hydraulic change-over valve set A consisting of hydraulic change-over valves 8, 9, 10 and 11 for an actuator by way of a pipe line 5; the second hydraulic pump 3 supplies discharge oil under pressure to another hydraulic change-over valve set B consisting of hydraulic change-over valves 12, 13, 14 and 15 for operation of an actuator by way of another pipe line 6; and the pilot pump 4 supplies discharge oil under pressure pressure oil to an operation system including a pilot valve 16 by way of a further pipe line 7. If the pilot valve 16 is operated and consequently a pressure signal is generated simulataneously in pilot oil chambers of the hydraulic change-over valves 11 and 15, then the hydraulic change-over valves 11 and 15 are changed over so that discharge oil under pressure of the first hydraulic pump 2 now flows into the hydraulic change-over valve set A by way of the pipe line 5 and then further into a pipe line 20 or 21 by way of a changed over path of the hydraulic change-over valve 11 while discharge oil under pressure of the second hydraulic pump 3 flows into the hydraulic change-over valve set B by way of the pipe line 6 and then into a pipe line 22 or 23 by way of a changed over path of the hydraulic change-over valve 15. Here, the pipe lines 20 and 21 are connected intermediately thereof to the pipe lines 22 and 23 and then to a head side oil chamber and a rod side oil chamber of a hydraulic cylinder 17 for the arm of the hydraulic shovel, respectively. Accordingly, the discharge oil pressure of the first hydraulic pump 2 and the discharge oil under pressure of the second hydraulic pump 3 are added to each other to operate the hydraulic cylinder 17.
As should be understood, change-over valves 8 to 15 are directional control valves with a flow rate regulating function, and valve 8 is a control valve for a hydraulic motor serving as a running actuator. Valve 9 is a control valve for the joining of an actuator for a boom and valve 10 operates as a control valve for an actuator for the swinging motion. Valve 11 is a control valve for an actuator for an arm and valve 12 serves as a control valve for the joining of a hydraulic motor serving as a hydraulic actuator for running operation. Valve 13 is a control valve for the actuator for the boom while valve 14 operates as a control valve for an actuator for a bucket and valve 15 is a control valve for joining of the actuator for the arm.
The arm and the boom are required to have different operating speeds depending upon the kinds of excavating operations being performed. When a high speed is required, oil under pressure from the pump 2 and oil under pressure from the pump 3 are joined together and supplied to the actuators. In FIG. 2, only the actuator 17 for actuating the arm b is shown while the other actuators are omitted. As can be seen from FIG. 2, the change-over valve set B includes, on the upstream side of the control valve 15 for joining of the arm actuator, control valve 13 for the boom actuator and control valve 14 for the bucket actuator, and oil under pressure is supplied from the hydraulic pump 3 commonly to the control valves 13 and 14.
Control valves 11 and 15 are controlled simultaneously by a single signal from a remote control valve 16 of the pilot type.
In the case of operation as seen in FIG. 3, the load is low, but fine operational control is required. In particular, manual operations for moving the arm, boom and bucket simultaneously and slowly are required. Thus, if the pilot remote control valve 16 is manually operated to control the control valves 11 and 15 for the arm actuator to reduce the flow rate of pressurized oil to flow therethrough, then excessive pressurized oil will flow into the control valve 13 for the boom actuator and the control valve 14 for the bucket actuator located on the upstream side of the control valves 11 and 15, while wil change the operating speeds of the boom and the bucket so as to deteriorate or disable fine control of them.
When it is intended to perform an operation by use of the hydraulic cylinder of a converging hydraulic circuit as described above, for example, when it is intended to operate an arm b of such a hydraulic shovel as shown in FIG. 3 to perform a shaping operation of a vertical or inclined surface d or a light excavating operation such as leveling of the ground, an operation lever for operating the arm b must be operated in a delicate manner and at a speed conforming to a speed of movement of a boom a or a bucket c or a turning motion. Accordingly, operability is low, and much skill is required for operation.